Electrospray is a technique for dispersing a liquid to produce an aerosol. In this technique, a liquid is supplied through a capillary and a high voltage is applied to the tip of the capillary. There is also provided a plate biased at low voltage, such as ground, spaced apart from the capillary in a direction normal to the capillary. The relatively high potential at the tip results in the formation of a Taylor cone. A liquid jet is emitted through the apex of the cone. The jet rapidly forms into droplets as a result of Coulomb repulsion in the jet as shown in FIG. 1.
FIG. 2 shows the related technique of electrospinning. Similarly to electrospray, a voltage source is connected between the tip of a capillary 1 and a collector plate 2. Again, as a result of Coulombic repulsion and surface tension forces a Taylor cone forms. If the liquid is a polymer or other liquid with a viscosity which is high enough (due to high molecular weight), the liquid jet emitted from the Taylor cone does not break up. The jet is further elongated by electrostatic repulsion in the polymer or liquid until a thin fibre is produced. The fibre is finally deposited on the collector 2. Instabilities in the liquid jet and evaporation of solvent can cause the fibre not to be straight and may curl. By careful choice of polymer and solvent system combined with a high enough electric field, fibres with nanometer scale diameters can be formed.
The electrospinning process is a particularly versatile process for the productions of micron-scale fibres and nanofibres. Materials such as polymers, composites, ceramic and metal nanofibres have been fabricated directly or through post-spinning processes. Diameters of 3-1000 nm have been achieved. The fibres produced can be used in a diverse range of fields, from scaffolds for clinical use, to nanofibre mats for sub-micron particulate filtration. Attempts have been made to fabricate more complex fibres, such as fibres having a core material different to an outer shell material, and fibre materials incorporating drugs in the outer shell or bacteria and viruses in the inner core. This process is known as coaxial electrospinning or co-electrospinning.
In prior art co-electrospinning techniques, such as that shown in FIG. 3, a solution 10 to form a core and solution 20 to form a shell are delivered through concentric openings in a nozzle 40. An electric field is applied to the nozzle by a voltage supply 30 to draw the shell solution into a Taylor cone 50 where the electrostatic forces within the surface of the shell solution overcome surface tension forces and a jet 60 issues from the cone. The relative viscosities of the core and shell solutions will also affect the formation of the coaxial jet. Frictional forces such as viscous dragging between the core and shell solutions will cause the core solution to be pulled into the Taylor cone and jet along with the shell solution. Similar to single component electrospinning, a bending instability in the jet will cause the jet to spiral 70. As the jet is drawn to the collector 80, electrostatic forces in the outer surface of the shell fluid will cause the jet to lengthen and thin. Solvents in the core and shell solutions will evaporate so as the jet thins it will begin to solidify into a core-shell fibre. Under continuous operation gram-scale quantities of fibre can be produced rapidly.
Prior art techniques have suggested that the structure of core-shell fibres are influenced by the humidity of the electrospinning environment. However, it would be desirable to provide a method of reliably electrospinning nanotextured core-shell fibres and controlling the texture of electrospun core-shell fibres.